The present invention relates to a lubricating composition for use in hot rolling of various steels including carbon steels and stainless steels. More particularly, it pertains to a lubricating composition which can exert lubricating properties, such as prevention of galling and reduction of roll wear, not only in sheet rolling but also in caliber rolling into shapes, rods, and tubes. The lubricating composition of the present invention has very high lubricity and is particularly suitable for use in hot rolling of stainless steels which are highly susceptible to galling. The present invention also relates to a lubricating method using such a composition.
The corrosion resistance of steels can be drastically improved by addition of a relatively large amount of chromium. High-Cr steels, typical of which are stainless steels, contain 13% by weight or more of chromium and form a stable chromium oxide protective film on the steel surface, thereby passivating the surface and improving the corrosion resistance. However, the surface oxide film is much thinner than that formed on the surface of carbon steels and is readily removed upon plastic deformation during hot rolling.
Also, in hot rolling of carbon steels under a high load, e.g., hot rolling at a relatively low temperature or a high reduction ratio, the surface oxide film does not sufficiently protect the steel surface or it is readily removed upon severe plastic working.
Hot rolling of a steel under these circumstances often causes seizing of the steel on the work rolls, resulting in a roughening of the surfaces of the work rolls, which, in turn, leads to the formation of surface flaws (hereinafter referred to as "seizure flaws") on the hot-rolled product. In addition, the work rolls wear so rapidly that the pass schedule may be limited.
The fragments of oxide film removed from the steel may remain on the steel surface as hard foreign matter, which is introduced into the roll gaps in subsequent or downstream mill stands and may cause the formation of surface flaws (hereinafter referred to as "scale flaws") on both the rolls and the hot-rolled product.
The formation of these surface flaws, which is generally called galling, is a serious problem in hot rolling. Any appreciable surface flaws must be removed by dressing the hot-rolled product by means of grinding, for example, or if the flaws are severe, the hot-rolled products have to be scrapped.
In order to cope with this problem, a lubricant is usually applied to the work rolls or their backup rolls in order to reduce the friction between the work rolls and the steel, thereby preventing seizure and hence minimizing surface roughening and wear of the rolls and improving the quality of the hot-rolled product.
One such lubricant proposed in Japanese Unexamined Patent Application Kokai No. 47-18907(1972) is a lubricating composition which comprises a natural fatty acid, a minor amount (0.1% -10% by weight) of a water displacing agent, and optionally a mineral lubricant oil. The water displacing agent used in the composition is preferably an oil-soluble sulfonate salt such as a metal petroleum sulfonate.
Japanese Patent Publications Nos. 62-14598(1987), 62-39198(1987), and 62-39199(1987) describe lubricating compositions comprising finely divided calcium carbonate of 10 .mu.m or less in size dispersed in water or a lubricant base oil.
However, these lubricating compositions are designed to be used in hot rolling of carbon steels under normal conditions and cannot prevent stainless steels from galling during hot rolling. Therefore, surface flaws are formed on the hot-rolled stainless steels and the work rolls used in the hot rolling wear rapidly
Lubricating compositions which have been proposed for use in hot rolling of stainless steels comprise an iron oxide powder dispersed in a lubricating oil, as described in Japanese Unexamined Patent Application Kokai No. 63-254195(1988), or a graphite powder dispersed in a viscous aqueous solution, as described in Japanese Unexamined Patent Application No. 1-167396(1989). The use of an iron oxide powder, however, does not adequately prevent galling or greatly reduce roll wear during hot rolling of stainless steels. Graphite brings about an extreme decrease in the coefficient of friction and may cause the stainless steels to slip or fail to smoothly insert into the roll gap with tight engagement. Therefore, graphite cannot be used in an amount sufficient to completely prevent the rolls from galling and significantly reduce the roll wear.
These prior-art lubricating compositions are mainly intended to prevent the formation of seizure flaws. No effective measures have been established with respect to prevention of scale flaws caused by foreign matter and dirt remaining on the steels to be hot rolled.